Recent studies on the mutagenicity of chemical carcinogens in various test systems have shown that there is a positive correlation between mutagenicity and carcinogenicity. Our previous studies with chemical carcinogens in Neurospora crassa not only show a positive correlation, but also indicate that potent chemical carcinogens induce predominantly base-pair substitution mutations. It is necessary to extend our studies to other carcinogenic compounds to determine whether these compounds are also mutagenic in Neurospora and to determine whether these compounds induce base-pair substitution mutations. The ad-3 test system of N. crassa is used in these studies. Conidia from a genetically marked two-component heterokaryon are harvested and are treated with chemical carcinogens. The treated and untreated conidia are analyzed for the presence of ad-3 mutants by the direct method. The isolated ad-3 mutants are made homokaryotic for the adenine requirement and characterized by a series of simple tests.